Valves designed for use in spacecraft must endure great environmental extremes. The valves must be extremely reliable, yet must be lightweight. Such valves experience much shock and vibration during launch of the spacecraft, as well as extremes in temperature. The fluids the valves control are often extremely corrosive, requiring the use of special materials. Seals are particularly vulnerable since they are often made from resinous or elastomeric material that is subject to swelling or deteriorates when exposed to these fluids.
Dual-fuel thruster systems for spacecraft typically use corrosive fuels such as hydrazine, monomethylhydrazine, and nitrogen tetroxide, an extremely unfriendly substance that is incompatible with most materials used to make valve seals. Tetrafluoroethylene, sold under the trade name TEFLON by DuPont, is unaffected by nitrogen tetroxide, however, making TEFLON a strong candidate for use in the manufacture of valve seals. Unfortunately, TEFLON has other properties that make it difficult to use in an environment in which parts cannot be replaced.
TEFLON is not resilient and has no shape memory. As a result, deformations and damage are permanent and cumulative, leading to leakage when TEFLON is used to make seals and/or sealing valve seats. One source of deformation is the shock and vibration of launch. Deformations also can result from engagement between a poppet and a TEFLON valve seat.
Additional properties of TEFLON that cause problems are its tendency to flow under load ("cold flow") and its high coefficient of thermal expansion. Any load applied to a TEFLON valve seat can cause deformations of the seat and result in leakage. The high coefficient of thermal expansion results in sensitivity to changes in temperature. Even a small change in temperature can result in expansion or shrinkage of a TEFLON valve seat that can cause leakage.
Thus, while valve designers use TEFLON valve seats because of its compatibility with corrosive chemicals, its other properties render such use problematic. There is a need for a TEFLON valve seat arrangement that minimizes the likelihood of deformation of the seat from shock, vibration, and cold flow. There is also a need for a TEFLON valve seat that limits exposure of the seat to changes in temperature that can cause undesirable expansion and contraction of the seat. Further, there is a need for a valve seat that reduces or eliminates leakage ordinarily resulting in the natural deformation caused by contact between the poppet and the seat.
Most prior art valves using TEFLON as a sealing material hold the TEFLON parts in place with threaded retainers. As they are rotated during assembly, the threaded parts slide against the TEFLON parts, causing pitting, stretching, and other deformations of the TEFLON parts, increasing the possibility of leakage through the seal. Additionally, the assembled threaded parts place a load on the TEFLON, causing cold flow of the TEFLON that can lead to improper mating of parts and further leakage. While the amount of leakage caused by such damage and cold flow may be acceptable in the applications for which these prior art valves are designed, such leakage is not acceptable in dual-fuel thruster systems for spacecraft. Consequently, there is a need for a sealing valve seat arrangement using TEFLON that is not assembled with threads and that allows extremely low leakage.
While an interference fit would be preferred to avoid damaging the TEFLON seat, the properties of TEFLON cause problems when such an interference fit is actually attempted. For example, when pressing the TEFLON into position, the TEFLON will tear at the interface between the metal and the TEFLON, resulting in leakage paths. The annealing temperature for TEFLON is on the order of 400.degree. F., and TEFLON has a large coefficient of thermal expansion. These properties make it easy to expand the TEFLON so that it can be shrunk-fit onto a metal piece. However, to achieve a similar interference fit of a second piece of metal over the TEFLON seat, the metal piece must be heated to a temperature ranging from 420.degree. F. to 600.degree. F., depending on the exact type of metal used to make the metal piece and the desired amount of interference. Thus, the second metal piece would be heated beyond the annealing temperature of the TEFLON seat, and possibly beyond its melting point, altering the properties of the seat, if not outright destroying it. There is a need, therefore, for a method of making a valve seat arrangement using a TEFLON seat where the parts are assembled using an interference fit but without exposing the seat to damaging heat.
Additionally, with most prior art arrangements, a small indentation results when the poppet contacts the seat. If the poppet is not correctly guided, the poppet will create overlapping indentations, resulting in leakage. Thus, there is a need for a valve seat arrangement in which the poppet is properly guided to prevent the formation of such overlapping indentations.